This grant proposal aims to identify the peptides that mediate T cell help for dsDNA-specific B lymphocytes in lupus. Systemic lupus erythematosus (SLE) is a disease of unknown etiology. Female (NZB x NZW) F1 (abbreviated B/W) mice recapitulate most of the salient characteristics of human lupus patients. Among these characteristics are serum antibodies to dsDNA, which are specific for the clinical diagnosis in a majority of SLE patients, as well as in all older B/W mice. An important question, therefore, is what drives production of high-affinity anti-dsDNA antibody? In the B/W mouse, the generation of high-affinity anti-dsDNA antibodies of IgG class is absolutely T-helper- cell dependent. dsDNA by itself is not an antigen in either normal or B/W mice, and DNA is not bound and presented by MHC molecules to T helper cells. The prevailing hypothesis is that DNA-protein complexes such as nucleosomes mediate the generation of high-affinity IgG antibodies. But the identity of the physiological B cell antigen is unknown. Specifically, there are no peptides known that are presented by dsDNA-specific B cells from any lupus-prone mouse model. As a result, it is not known how T cells help induce high-affinity anti-dsDNA antibodies. To reduce the complexity of the autoimmune response, we will make use of a classic lupus mouse model that has been modified by transgenesis in a conceptually simple way. It yields a quasimonoclonal auto- immune response to (only) dsDNA, while leaving the kinetics of the response intact. Unlike the germinal center B cells in old wild-type B/W mice, with various antigenic specificities, the majority of the B cells of the old transgenic mice have identical and monoclonal dsDNA specificity with high affinity. We will sort the germinal center B cells, and elute and identify th peptides bound to their class II molecules. We will then test these peptides for their ability to stimulate T cells of the B/W mice and for their function in the production of anti-dsDNA antibody. This will indicate which antigen is internalized by the immunoglobulin receptor of B cells with high affinity to dsDNA.